"We found shell casings. We heard people had been shot, but there were no victims on scene so it was kind of the beginning of the investigation," Penn Hills Police Chief Howard Burton said.

The owner, Mary Sullivan, said she saw a vehicle pull in backward, and its engine was left running when at least one person got out and fired shots from outside the fence. The car sped away immediately after the shots rang out.

"As soon as those gunshots went off, everyone from this side of the pool ran to the other end, away from the sound of the gunshots," manager Stephen Kustra said.

Police say the victims, all males, drove themselves to UPMC East. Two were then flown to UPMC Presby, while the third taken by ambulance.

"There were multiple gunshot wounds to the individuals," Burton said. "One had at least two. One had possibly three. Another one had one or two."

Police found shell casings from two different weapons on scene. Police said they haven't had many problems at the pool in the past.

"Most of the parties are uneventful and great parties, and this was just one that wasn't," Sullivan said.

Police say they aren't exactly sure what happened, but that a DJ had been playing during a private party at the large outdoor pool.

"All that I could make out of most of it was that the shooter came for vengeance. He was after one of the individuals that was shot," Sullivan said.

Police said the investigation is still in its early stages and it's hard to say if someone was being targeted.

"With what's going on today, people have grudges against other people for whatever reason, and they hear they show up and then there's gunshots fired," Burton said.